I saw your question in the Google+ group of Antergos. I can only answer the one about the multilib repo.
You mentioned that you ve installed the i 686 version (meaning the 32bit).
Multilib repo “allows the user to run and build 32-bit applications on 64-bit installations of Arch Linux…”https://wiki.archlinux.org/index.php/multilib
so it is normal not finding it in your system (if I remember well, Cnchi installs the multilib repo on 64bit systems)…
Sorry for not being able to help with Steam. I don t play…

@Wyn, I was NOT talking about which repo Steam is in, but answering the question why there was not a multilib repo in the system. I found the question in the Google+ group, I answered it here.
As for Steam, I really don t mind where it is in either bit version. I have never played, I never will…